1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly, to a cathode ray tube to reduce a doming effect of a shadow mask which causes a color purity of a screen to be deteriorated.
2. Description of the Background Art
Generally, a cathode ray tube is a device for converting an electric signal into an electron beam and emitting the electron beam to a fluorescent surface to realize a screen. Such a cathode ray tube is widely used since a display quality for the cost of the cathode ray tube is excellent.
The cathode ray tube will be explained with reference to attached drawings.
FIG. 1 is a schematic view showing one example of the cathode ray tube.
As shown in FIG. 1, the cathode ray tube comprises: a panel 3 of a front glass formed to have a flat outer surface and an inner surface of a predetermined curvature; a funnel 2 of a rear glass engaged to the panel 3 for forming a vacuum space; a fluorescent surface 13 deposited on an inner side surface of the panel 3 as a luminant; an electron gun 6 for emitting an electron beam which illuminates the fluorescent surface 13; a deflection yoke 7 mounted at an outer circumferential surface of the funnel 2 with a predetermined interval for deflecting the electron beam 5 to the fluorescent surface 13; a shadow mask 8 installed with a constant interval from the fluorescent surface 13; a mask frame 9 for fixing and supporting the shadow mask 8; and an inner shield 10 extending from the panel 3 to the funnel 2 for shielding external terrestrial magnetism in order to prevent color purity from being deteriorated by the magnetism.
Also, a spring supporter 14 where a supporting spring 11 which supports the mask frame 9 to the panel elastically is fixed at the inner side of the panel 3, and a reinforcing band 12 for dispersing stress generated at the panel 3 and the funnel 2 is installed at the outer circumferential surface of the panel 3.
The shadow mask 8 is a color selection device in which the electron beam 5 emitted from the electron gun 6 selectively strikes the fluorescent surface 13 deposited on the panel 3. As shown in FIG. 2, the shadow mask comprises an effective surface 17 having a plurality of electron beam passing holes 15 at a center thereof; an ineffective surface 19 formed at a periphery of the effective surface 17 and not having the electron beam passing holes 15; and a skirt portion 21 extended from the ineffective surface 19 towards a vertical direction thereof and fixed to the mask frame 9.
In the shadow mask 8, the skirt portion 21 is fixed to a side surface of the mask frame 9 by spot welding and etc., and arranged to be adjacent to the fluorescent surface 13 of the panel 3 by fixing the mask frame 9 to the panel 3.
A welding point 25 by which the shadow mask 8 is welded to the mask frame 9 is located at a middle portion of a long side portion and a short side portion of the skirt portion 21, and two slots 23 are respectively formed at outer sides of the welding point 25.
In the conventional cathode ray tube, the electron beam 5 emitted from the electron gun 6 is deflected by the deflection yoke 7, passes through the plurality of electron beam passing holes 15 formed at the shadow mask 8, is landed on the fluorescent surface 13 formed at the inner surface of the panel 3, and illuminates each luminant, thereby realizing a screen.
At this time, a part of the electron beam 5 does not pass through the electron beam passing holes 15 of the shadow mask 8 but collides with the shadow mask 8. By the collision of the electron beam 5, heat of a high temperature is generated at the shadow mask 8.
Accordingly, the shadow mask 8 is distorted by the collision heat of the electron beam 5, which is referred to as a doming effect.
The doming effect displaces the electron beam passing holes 15, which generates a mis-landing, in which the electron beam 5 does not land at a proper fluorescent surface, thereby causing a color speck on the screen. Also, since the electron beam passing holes 15 of the shadow mask 8 are very small, the color speck on the screen is generated even with only a slight doming effect.
Causes of the doming effect include heat expansion of the shadow mask 8 by collision heat of the electron beam 5 and a distortion of the shadow mask 8 by heat expansion of the mask frame 9. The doming effect of the shadow mask 8 will be explained with reference to FIGS. 3, 4, and 5.
First, the doming effect caused by heat expansion of the shadow mask 8 by collision heat of the electron beam 5, as shown in FIG. 3, is generated resulting from the shadow mask 8 being heat-expanded by heat of approximately 80˜100° C. generated at a time when a part of the electron beam 5 collides with the shadow mask 8 when a power source is applied to the cathode ray tube.
By the doming effect of the shadow mask 8, the electron beam passing holes 15 of the shadow mask 8 are displaced. According to this, a landing position of the electron beam 5 is changed by ΔA, and by the mis-landing of the electron beam 5a, a color purity of a screen is deteriorated.
In the meantime, as shown in FIG. 4, the doming effect of the shadow mask 8 by heat expansion of the mask frame 9 is generated as collision heat of the electron beam 5 is transmitted to the mask frame 9 to expand the mask frame 9, and the expanded mask frame 9b pulls the shadow mask 8a. 
Accordingly, a curved surface of the shadow mask 8b is changed to displace the electron beam passing holes 15, so that the landing position of the electron beam 5 is displaced by ΔB and a mis-landing in which the electron beam 5b does not land to a proper fluorescent surface 13 is generated, thereby deteriorating a color purity of a screen.
As shown in FIG. 5, amount of the mis-landing ΔB generated by the doming effect by the heat expansion of the mask frame 9 is greater than that ΔA generated by the doming effect by the heat expansion of the shadow mask 8, and directions of the mis-landing are different.
Also, the mis-landing ΔA by the heat expansion of the mask frame 9 is for a longer time than the time of the mis-landing ΔB by the heat expansion of the shadow mask 8.
That is, the doming effect of the shadow mask 8 by heat expansion of the mask frame 9 influences a color purity of a screen to a greater degree than the doming effect by heat expansion of the shadow mask 8.
The doming effect of the shadow mask 8 by the heat expansion of the mask frame 9 is generated as the heat-expanded mask frame 9 pulls the skirt portion 21 of the shadow mask 8. As shown in FIG. 6, a part to which a tensile force of the mask frame 9 is applied is near the welding point 25 of the mask frame 9 and the shadow mask 8 (an oblique line part), and a direction of the tensile force of the mask frame 9 is equal to that of an arrow.
In the meantime, in order to attenuate the tensile force, notches 23 are formed at both sides of the welding point 25 of the skirt portion 21 to reduce an influence of the tensile force on the entire skirt portion 21.
However, in accordance with that effort, as the size of a cathode ray tube becomes larger, a size of the shadow mask and the mask frame also becomes larger. According to this, the tensile force by the heat expansion of the shadow mask and the mask frame becomes great, thereby having a limited ability to attenuate the tensile force only by the notches. Therefore, a technique to attenuate the tensile force generated near the welding points of the skirt portion of the shadow mask is required.